Partial exclusion of bacteriophage T2 by bacteriophage T4: an exclusion-resistant mutation in gene 56 of T2

The early genes of bacteriophage T2 are partially excluded from the progeny of crosses between the related bacteriophages T2 and T4. This is due to complete exclusion from the progeny of six exclusion-sensitive sites in T2. A mutation [exr(56)l] in the sensitive site near T2 gene 56 renders the site partially resistant against exclusion. This paper describes the mapping of the exr(56)l mutation. The mutation was mapped between two clusters of amber 56 mutations in T2, but mapping was not completely unequivocal. Additional evidence for location of exr(56)l within gene 56 was provided by the decrease in the activity of the gene 56 product (dCTPase: EC 3.6.1.12) induced by T2 exr(56)l strains. The location of exr(56)l within an essential gene contradicts the exclusion model proposed by Russell & Huskey (1974)